The deterioration of a rechargeable battery relates to the amount that one or more performance characteristics of the battery have degraded over time. The battery performance characteristics that are subject to deterioration include, for example, the charge capacity of the battery and the battery's equivalent series resistance (ESR). The charge capacity of the battery is a measure of the amount of energy that may be stored in the battery when fully charged. The ESR of the battery is relevant to determining the amount of current that the battery is capable of delivering. It is desirable in many systems and devices to monitor the deterioration of one or more battery performance characteristics, for instance, to determine when the rechargeable battery should be replaced.